Complement activation by human IgG antibodies to galactose-α-1,3-galactose

Some human antibodies may paradoxically inhibit complement activation on bacteria and enhance pathogen survival in humans. This property was also claimed for IgG antibodies reacting with terminal galactose-alpha-1,3-galactose (Gal alpha 3Gal; IgG anti-alpha Gal), a naturally occurring and abundant antibody in human plasma that targets numerous different pathogens. To reinvestigate these effects, we used IgG anti-alpha Gal affinity isolated from a pool of normal human IgG and human hypogammaglobulinaemia serum as a complement source. Flow cytometry was performed to examine antibody binding and complement deposition on pig erythrocytes,Escherichia coliO86 andStreptococcus pneumoniaeserotype 9V. Specific nanobodies were used to block the effect of single complement factors and to delineate the complement pathways involved. IgG anti-alpha Gal was capable of activating the classical complement pathway on all the tested target cells. The degree of activation was exponentially related to the density of bound antibody onE. coliO86 and pig erythrocytes, but more linearly onS. pneumoniae9V. The alternative pathway of complement amplified complement deposition. Deposited C3 fragments covered the activating IgG anti-alpha Gal, obstructing its detection and highlighting this as a likely general caveat in studies of antibody density and complement deposition. The inherent capacity for complement activation by the purified carbohydrate reactive IgG anti-alpha Gal was similar to that of normal human IgG. We propose that the previously reported complement inhibition by IgG anti-alpha Gal relates to suboptimal assay configurations, in contrast to the complement activating property of the antibodies demonstrated in this paper.